Petrology of ijolites from the Prairie Lake carbonatite complex

Abstract

This study investigates the major and trace element composition of minerals of the ijolite series rocks occurring at the Prairie Lake carbonatite complex, northern Ontario, together with comparative data for ijolites from the Fen complex, Norway. Trace element data (Sr, Zr, REE) were collected by LA-ICP-MS for clinopyroxene, garnet, and apatite. and in conjunction with the major element data are used to develop a petrogenetic model for Prairie Lake. The ijolites and calcite ijolites (hollaites) of Prairie Lake carbonatite complex have been formed by magma mixing, crystal settling, solid-state deformation, and deuteric alteration. The complex represents at least three stages of intrusion by melts of differing composition. The initial stage is predominantly biotite pyroxenites and associated coarse carbonatite veins. The second stage is primarily members of the ijolite series together with solid state deformation creating meta-ijolites, with differentiation forming malignites (potassic nepheline syenites). The third major stage is the intrusion of the CII carbonatites derived from different batches of magmas. These latter rocks contain xenoliths of ijolite suite rocks, wollastonite apatitite and phoscorite. Pyroxene compositions show an evolutionary trend from diopside in biotite pyroxenites to Fe-enriched diopside-augite in ijolites to aegirine in malignites. Clinopyroxene major and trace element data shows that clinopyroxene cores from the biotite pyroxenites formed at depth and were emplaced as part of a later event. These data are used to show that a continuously filled fractionating magma chamber was not present at Prairie Lake and that the complex formed as result of small intrusions of nephelinite magma into pre-existing ijolites. A similar style of petrogenesis is suggested for the Fen complex.